1. Field of the Invention
This invention relates in general to a vehicle speed control apparatus and in particular to a speed control apparatus which utilizes vehicle speed and engine load feedback to maintain the vehicle at an operator selected desired road speed.
2. Description of the Prior Art
Early devices for controlling the speed of a vehicle were typically comprised of complex mechanical controls having a large number of interrelated moving parts. These mechanical speed controls were expensive to manufacture and difficult to maintain in an accurate operating condition.
The problems associated with the mechanical devices were overcome by the use of an electronic control device. At a comparable cost, the electronic device could be made more accurate and more reliable than the mechanical device.
Early electronic speed controls were typically closed-loop systems having a single feedback loop. In this type of control system, a feedback signal representing the actual vehicle speed was compared to a signal representing the desired vehicle speed to control the vehicle throttle and maintain the vehicle speed at the desired value. However, these single feedback loop systems presented several problems with respect to hunting, overshooting and undershooting of the control system.
One attempt to minimize the hunting, overshooting and undershooting effects of single feedback loop systems was to provide a second feedback loop which monitored the position of the vehicle throttle in order to anticipate changes in vehicle speed. A feedback signal, representing the throttle position, was utilized in conjunction with the actual speed signal and the desired speed signal to maintain the vehicle speed at the desired value without undue oscillations and hunting effects. Such control systems are disclosed in U.S. Pat. Nos. 3,575,256; 3,648,798; and 3,705,639.
However, this type of control system had some associated problems. The system typically required a potentiometer to be connected to the vehicle throttle linkage for monitoring the throttle position. Thus, the correct adjustment of the throttle linkage was an important requirement.
Another attempt to minimize the hunting, overshooting and undershooting effects was a speed control system disclosed in U.S. Pat. No. 3,820,624. In this system, a slightly delayed synchronous pulse corresponding to the actual vehicle speed signal was compared with an actual speed pulse converted from the actual speed signal. A speed acceleration-deceleration compensating circuit temporarily adjusted the desired vehicle speed signal in response to the width difference between the synchronous pulse and the actual speed pulse. This temporary adjustment reduced overshooting and undershooting of the control system. However, this system was relatively complex and therefore expensive to manufacture.